In the laying of supply and waste pipes above all in areas within towns, technical and economic requirements resulting from the presence of sensitive living spaces, dense mains systems and much used highways have led to the fact that the closed construction method relative to the open construction method is continually gaining in importance. Pipe advance counts amongst the most important construction methods in this respect for trenchless laying of sewers, water pipes, protected and jacketed tubes, ducts and conduits.
In this method, advance pipes are driven through the subsoil into a target shaft starting from a starting trench with the help of a main pressing station and assistance of intermediate pressing stations. The advance in straight or curved line guidance is thereby made possible by a controllable shield tunnel boring machine which is disposed in front of the first pipe. The stone is broken up at the front location mechanically over a partial or full surface area and is conveyed away to above ground by the advanced piping. As a function of the outer diameter there are used advance pipes premanufactured in a factory, as a rule with a circular annular cross section 1200≦DN/ID≦3000, or advance pipes produced in a factory on site with even greater nominal width.
Pressing-in of shield tunnel boring machine and piping is effected with the help of the advance force produced by the pressing station. It serves for overcoming the penetration resistance of the bore head into the existing subsoil and the frictional resistances along the peripheral surface area of the shield tunnel boring machine and also the subsequent piping in the ground (surface friction). In order to be able to control the advance in the vertical and horizontal direction, the dimension of the borehole produced by the advance machine is slightly greater than the outer diameter of the advance pipes so that these can be angled off relative to each other within the borehole and hence the pipes which are connected to each other in an articulated manner to form piping can follow the direction changes forced by the shield tunnel boring machine without or with only slight jamming. The dimension by which the borehole radius exceeds the pipe outer radius is termed annular gap (or annular space from a spatial point of view). The annular space is generally, in particular when advancing in unstable loose stone, filled with a so-called supporting and lubricating agent which, on the one hand, supports the soil from falling into the annular space and in addition reduces the required advance forces because of its friction-reducing effect between ground and pipe outer surface. For this purpose, the supporting and lubricating agent is under pressure, the level of which is dependent in particular upon the horizontal and vertical earth or soil pressure, the groundwater pressure, the permeability of the soil, the annular space dimensions and also the rheological properties of the supporting and lubricating agent.
A problem which occurs frequently in practice is presented by supporting and lubricating agent losses and pressure drops which go beyond a tolerable amount, with significant consequences for the further course of the advance operations as far as advance standstill or impermissible stressing of the pipes.
During lubrication and supporting of the annular space, liquid, solid-free and solid-containing liquids (so-called drilling fluids) are used nowadays, in particular water, bentonite drilling fluids or bentonite-polymer drilling fluids.
The supporting is achieved with the help of a corresponding pressure impingement of drilling fluid, it requiring to be ensured that the pressure of the supporting liquid at any place to be supported must be greater than the pressure due to the groundwater and subsoil.
In stable, groundwater-conducting subsoil, the supporting agent pressure need merely counteract the prevailing groundwater pressure. For this application case, all the previously mentioned drilling fluids are suitable.
In unstable subsoil, the respectively used supporting agent must develop a mechanism with the ground to be supported which makes it possible to transmit in full the difference of the pressure of the supporting agent and of the prevailing earth and/or groundwater pressure to the grain structure of the existing ground. For this application case, in particular solid-containing supporting agents, such as e.g. bentonite drilling fluids and bentonite-polymer drilling fluids which have a corresponding flow limit, are suitable.
In the case of bentonite drilling fluids or bentonite-polymer drilling fluids, transmission of the pressure difference to the grain structure is effected in a time-independent manner if a zone is formed at the surface or up to a certain penetration depth in the region close to the surface of the borehole wall, the permeability of which zone is less than that of the existing ground. In this zone, the differential pressure between the supporting agent side and the soil to be supported is converted into an effective pressure acting on the grain structure.
At the interface between ground and bentonite drilling fluid or bentonite-polymer drilling fluid, a thin layer accumulates in addition in particular in the case of higher supporting agent pressure, said layer comprising superposed bentonite particles. This layer—also termed filter cake—seals the interface of the cavity wall and thus assists transmission of the supporting agent pressure to the grain structure.
The formation of impermeable zones or of the filter cake is however only successful when the bentonite particles which are dispersed in the bentonite drilling fluid or bentonite-polymer drilling fluid are larger than the smallest pores in the existing soil or freely occurring polymer particles block any still available pores mechanically-physically as a result of their moveability and plasticity.
The field of use of bentonite drilling fluids extends therefore to coarse-grain, loose to densely packed sand and gravels, non-uniform and non-homogeneous soils with water permeability coefficients of kf>10−3 m/s.
Bentonite-polymer drilling fluids comprise water as basic material to which bentonite and polymers are added. They are used preferably in coarse-grain soils with open structures, such as e.g. uniform coarse gravels in order to avoid outflows and in addition in clay soils which are inclined to swell and stick together. In practice, the most varied of types of polymers are used in the present application case. They thereby serve for example for filtrate reduction, as protective colloid and for viscosity regulation.
The problems of the existing method technique for supporting the annular gap and lubricating the piping are characterised by the following factors:
a) The modification of supporting agents with the help of additives and in particular polymers is essentially dependent upon the experience of the employees working on the building site. There are in fact corresponding operational guidelines for using polymers, such as e.g. the instructions W 116 of the DVGW (German Association of Gas and Water Applications), “Use of drilling fluid supplements in bore drilling fluids in boring works in groundwater” with corresponding dosage recommendations for the respective drilling fluid formulation; however because of the wide range of available types of polymers with different modes of operation, these should be regarded rather as generally valid handling recommendations. For the specific application case, in practice as a rule separate “experiments” are therefore implemented in order to identify suitable, modified supporting agents or self-formulated supporting agents are used. The danger exists however hereby that combinations of a plurality of polymers can lead to undesired reactions.
b) The automatic lubricating systems used to date inject continuously the previously established supporting and lubricating agent into the annular gap via lubricating stations integrated in the piping. Injection agent quantities and pressures for each lubricating station are thereby individually adjustable. Since, however, all the lubricating stations are supplied with the supporting and lubricating agent via a closed circulation from a single container or mixer installed in the region of the starting shaft, it is not possible, along the advance route, to react at the individual lubricating stations to changing geological structures with different supporting and lubricating agents which are suitable for the respective application case.
c) In loose, pebbly soils, the danger exists furthermore that the annular gap being dug collapses directly behind the shield tail over the piping. In this case there are no possibilities for producing the annular gap again. This situation must definitely be avoided since the surface friction increases abruptly and, upon reaching the pressing capacity, the danger exists of stalling of the advance.
In order to reduce the advance forces or surface friction, a microtunnel construction method was developed for use with groundwater in highly water-permeable soils, said method being used with the description “Support membrane depot box system”. This method is characterised in that, moving forward with the advance, an endless tube which is mounted in a magazine in the trailer of the advance machine and comprising soft PVC with a thickness of 0.3 mm is unwound, said tube enveloping the advance pipes. Parallel hereto the region between the tube and advance pipe and also between tube and exposed soil is injected under pressure with a bentonite suspension. There should be mentioned as disadvantages of this technology, which stand in the way of wider use, a very complex construction of shield tail and trailer, relatively large spatial requirement for the tube magazine and also the risk of damage to the tube.